BACKGROUND: An increase in late-onset myasthenia gravis (MG) has been reported. There are few large population-based studies over longer periods of time reflecting recent developments in MG incidence. METHODS: We identified a nationwide cohort of patients with incident myasthenia in Denmark in 1996-2009. We used a validated algorithm to track subjects based on a combination of diagnosis and prescription (pyridostigmine) data from nationwide registers. Patients with myasthenia were classified into early onset (